The defence of necessity in criminal law is where the defendant is arguing that it was necessary for them to commit a crime. For example, where a prisoner escapes from a burning prison he may raise the defence of necessity as it was necessary for him to escape. The defence of necessity often operates where the defendant has two alternatives either commit a crime or suffer or cause another extreme hardship. According to Sir James Stephen, there are three requirements for the application of the defence of necessity:

(i) the act is needed to avoid inevitable and irreparable evil;(ii) no more should be done than is reasonably necessary for the purpose to be achieved;(iii) the evil inflicted must not be disproportionate to the evil avoided.

The defence of necessity can only be pleaded in extreme circumstances and is often unsuccessful:

For a long time it was questioned as to whether the defence of necessity existed in English law. It was raised in the infamous case of Dudley & Stephens where it was again unsuccessful as it was held that the defence of necessity could not be raised for murder offences:

The defence of necessity has been more successful in medical cases. Whilst the language of necessity was not used in the case of Bourne, it is difficult to explain the outcome by means other than the defence of necessity:

The most liberal application of the defence was seen in the unusual case of Re A where the defence of necessity was allowed for the offence of murder in relation to a life saving operation to separate conjoined twins:

Whilst the defence of necessity is often used to protect medical professionals perceived to be acting in the best interest of their patients, the defence of necessity has been denied in self-medication cases involving cannabis:

Thus it can be seen that the defence of necessity is generally only successfully applied in medical cases. Outside of this the defence of duress of circumstances has largely taken over many cases which traditionally would have come under necessity. The defence of duress is still quite restrictive but is perhaps more amenable than the defence of necessity.